1. Field of the Invention
The present invention relates to a polytetrafluoroethylene (hereinafter referred to as PTFE) composition, a method for its production and a granulated product.
2. Discussion of Background
A composition having a filler blended to PTFE (hereinafter referred to as a PTFE composition) is widely used as a raw material for a molded product having improved creep resistance, abrasion resistance, etc. in addition to excellent properties of PTFE, such as heat resistance, chemical resistance and sliding properties.
A PTFE composition to be used for molding, is prepared by adding various fillers to fine powdery PTFE produced by a suspension polymerization method (hereinafter referred to as suspension-polymerized PTFE), followed by uniform mixing. Further, this PTFE composition may be granulated as the case requires to obtain a PTFE granulated product having a large particle diameter and good handling efficiency.
On the other hand, PTFE produced by a dispersion polymerization method (hereinafter referred to as a dispersion-polymerized PTFE) is mixed with an organic solvent such as naphtha to obtain a paste, which is then subjected to preforming. The obtained preform is subjected to extrusion molding (hereinafter referred to as a paste extrusion molding method). By the paste extrusion molding method, tubes or various film products are produced.
Heretofore, such dispersion-polymerized PTFE could not be subjected to commercially important molding such as compression molding or ram extrusion molding where suspension-polymerized PTFE was used.
The reason is considered to be as follows. Fine powdery PTFE produced by a dispersion polymerization method (hereinafter referred to as PTFE fine powder) has such a nature that upon receiving a shear force at the time of mixing with the filler, it tends to be fibrillated (hereinafter referred to as fibrillation). Accordingly, the dispersion-polymerized PTFE composition was likely to be lumpy, whereby handling efficiency for molding was poor. Further, a granulated product obtained from such a composition had a problem that not only the apparent density was low and the productivity in molding was low, but also cracks were likely to form in the molded product.
As a method for producing a uniform composition from a dispersion-polymerized PTFE and a filler, a method has been proposed in which an aqueous dispersion of the dispersion-polymerized PTFE is stirred with various fillers and coagulated (see e.g. JP-B-52-34653, JP-B-56-48528, JP-B-63-46097, and JP-A-8-253600). However, the obtained fine powdery PTFE composition and its granulated product had problems such that as PTFE was susceptible to fibrillation as mentioned above, they tend to be lumpy, their apparent densities were low, their yields as products were low, and cracks were likely to form in their molded products.
It is an object of the present invention to provide a dispersion-polymerized PTFE composition which can be subjected to molding, a method for its production and a granulated product thereof.
The present invention provides a PTFE composition comprising a dispersion-polymerized PTFE and a filler, wherein the powder irregular shape ratio of the PTFE is at most 50%.
Further, the present invention provides a method for producing the above PTFE composition, which comprises jetting high pressure water to a mixture comprising a dispersion-polymerized PTFE and a filler to obtain a PTFE composition having the PTFE and the filler dispersed.
Still further, the present invention provides a PTFE granulated product, obtained by granulating the above PTFE composition.